Common mode noise reduction in capacitive touch sensing

ABSTRACT

The present disclosure is directed to a system and method to remove common mode noise projected onto a touch sensor array from a display. The system is configured to activate two rows of electrodes at the same time, while coupling remaining rows of electrodes to ground. A first one of the two activated rows is used for detection of a touch and a second one of the two activated rows is used to detect common mode noise from the display. The common mode noise detected by the second row is removed from signals received from a plurality of columns of the touch sensor array.

BACKGROUND

Technical Field

The present disclosure is directed to a device and method to reducenoise in touch screen enabled liquid crystal displays.

Description of the Related Art

Touch screens have become ubiquitous in hand-held electronic devices. Auser interacts with a display on the electronic devices simply bytouching their finger or a stylus to a screen. Integrated with thedisplay are capacitive touch sensors that detect when the user has takenan action, such as selecting an icon. As components related to thedisplay become thinner and thinner, more and more noise interferes withthe sensors of the touch screen.

BRIEF SUMMARY

The present disclosure is directed to a thin electronic device thatincludes a touch sensor array adjacent to a display. The displayprojects parasitic capacitance to the touch sensor array. The parasiticcapacitance is consistent across the array, such that it is common toeach of the rows and columns of electrodes of the array.

To remove this parasitic capacitance the touch sensor array includesadditional circuitry coupled to one of the rows of electrodes duringnormal detection. This extra row is a noise detection and reduction row.

The circuitry includes a plurality of first current to voltageamplifiers, each one of the plurality of first current to voltageamplifiers is coupled to one the plurality of columns. The circuitryincludes a plurality of differential amplifiers, each one of theplurality of differential amplifiers is coupled to one of the pluralityof first current to voltage amplifiers. The circuitry also includes asecond current to voltage amplifier configured to be coupled to one ofthe plurality of rows, an output of the second current to voltageamplifier being coupled to each one of the plurality of differentialamplifiers.

This and other embodiments of the present disclosure are discussed inmore detail below.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, identical reference numbers identify similar elements.The sizes and relative positions of elements in the drawings are notnecessarily drawn to scale.

FIG. 1 is a stack of layers of an electronic device having a display anda touch sensor array;

FIG. 2 is a block diagram of circuitry associated with the display andthe touch sensor array;

FIG. 3 is a portion of the touch sensor array and associated circuitryaccording to an embodiment of the present disclosure; and

FIG. 4 is an alternative embodiment of the touch sensor array andassociated circuitry according to the present disclosure.

DETAILED DESCRIPTION

In the following description, certain specific details are set forth inorder to provide a thorough understanding of various embodiments of thedisclosure. However, one skilled in the art will understand that thedisclosure may be practiced without these specific details. In otherinstances, well-known structures associated with touch screen electronichave not been described in detail to avoid unnecessarily obscuring thedescriptions of the embodiments of the present disclosure.

Unless the context requires otherwise, throughout the specification andclaims that follow, the word “comprise” and variations thereof, such as“comprises” and “comprising,” are to be construed in an open, inclusivesense, that is, as “including, but not limited to.”

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment. Thus, the appearances of the phrases “in one embodiment” or“in an embodiment” in various places throughout this specification arenot necessarily all referring to the same embodiment. Furthermore, theparticular features, structures, or characteristics may be combined inany suitable manner in one or more embodiments.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contentclearly dictates otherwise. It should also be noted that the term “or”is generally employed in its sense including “and/or” unless the contentclearly dictates otherwise.

As used in the specification and appended claims, the use of“correspond,” “corresponds,” and “corresponding” is intended to describea ratio of or a similarity between referenced objects. The use of“correspond” or one of its forms should not be construed to mean theexact shape or size.

The present disclosure is directed to reducing parasitic capacitance(noise) detectable in an electronic device 100 that includes a touchsensor array 104, which originates from pixels in an associated display102. This parasitic capacitance occurs throughout the touch sensor array104 and occurs at each refresh of the display or in other words, everytime a pixel changes. The touch sensor array 104 includes a plurality ofrows and columns, for example, rows 204 and columns 206 in FIG. 3. Therows and columns may be referred to as transmitter channels and receiverchannels in this disclosure.

The parasitic capacitance is common to all of the transmitter andreceiver channels in the electronic device, in that the noise may bereferred to as common mode noise. This common mode noise is the sameacross each receiver channel (columns 206).

As the electronic device 100 is made to be thinner and thinner, thenoise that affects the touch sensor array increases. In some instances,the noise is so large that it can saturate the receiver channel outputs,overwhelming and saturating the system.

The present disclosure is directed to reducing or removing this noise orparasitic capacitance, giving manufacturers the opportunity to makeextremely thin displays. The aspect of the present disclosure may beutilized with flexible or bendable displays. These displays are movingfrom a spacing of 100 microns between the display and the touch sensorelectrodes to a spacing of 8 microns to 10 microns. This decrease indistance can increase the capacitance detected from noise 10 fold.

FIG. 1 is a simplified cross-sectional representation of layers of theelectronic device 100 that includes a display 102 integrated with atouch sensor array 104. The electronic device may be a cell phone,tablet computer, or any other device that allows a user to both viewinformation and interact with the information on a shared surface. Thisshared surface is often referred to as a touch screen or touch panel. Inparticular, the display is configured to display information, such as aphotograph, icons, or text. The touch sensor array 104 is a clear panelpositioned on the display such that the user can see the display. Thetouch sensor array 104 is configured to detect with electrodes, such aswith capacitive detection, when and where the user touches the display.

The touch sensor array 104 includes an electrode array layer 108 that iscovered by a glass or other protective clear layer 110. The glass layer110 may be 500 to 700 microns thick.

The display 102 includes an illumination panel 112, which is covered bya plurality of layers 114. The plurality of layers 114 will include avariety of layers depending on the properties of the display. Forexample, the display 102 in FIG. 1 is a liquid crystal display thatincludes a first polarizing layer 116, a thin-film transistor layer 118,a liquid crystal layer 120, an organic layer 122, an indium tin oxidelayer 124, a glass layer 126, and a second polarizing layer 128. Anoptically clear adhesive 131 is positioned between the second polarizinglayer 128 and the touch sensor 104.

As these layers for the display become thinner and thinner and differentmaterials and technologies are developed, noise from the display affectsthe sensitivity and output of the touch screen array. Some displays aremoving towards being flexible, extremely thin products, such thatreduction of noise from the close proximity of the display components tothe touch sensors is becoming increasingly important.

FIG. 2 is a representation of the interactions between a liquid crystaldisplay circuit 130 and the touch sensor 104, which result in noise orparasitic capacitance. The touch sensor 104 includes a touch sensorcontroller 105 that is configured to receive signals from the pluralityof columns 206 and the plurality of rows 204 of the touch sensor array200, referring to FIG. 3. The touch sensor controller 105 is alsoconfigured to activate and deactivate ones of the plurality of rows andcolumns in accordance with the present disclosure to detect capacitancefrom the user's touch and remove or reduce noise.

The display circuit 130 includes a variety of components, which are notdescribed in detail in this disclosure. The components include digitalto analog converters 134 associated with red, green, and blue colordisplay. Thin film transistors 132 are positioned between the digital toanalog converter and sub-pixel electrodes 136. Noise, or parasiticcapacitance, C_(SENSE) is generated between the sub-pixel electrodes 136and the touch sensor 104 such that capacitance is projected from thedisplay to the touch sensor 104. This parasitic capacitance createsinaccurate readings of capacitance. The parasitic capacitance isinversely proportional to the distance between the display and theelectrodes of the touch sensor, such that as the displays become smallerand thinner, the amount of parasitic capacitance increasessignificantly.

Anytime the display is updated or refreshed, noise is created betweenthe display and the touch sensor. For example, when pixels change to anew color, noise is created. In other words, capacitance based on thepixel change is detected by the touch screen array, which can createfalse capacitance (an unintended touch may be detected).

As noted above, new displays are becoming more thin, increasing thepixel density, and the overall number of pixels, so the switching noiseas the larger number of pixels are changed is more significant. Noisecaused by the pixels is basically uniform across the display (commonmode). The present disclosure is directed to improving the signal tonoise ratio of these electronic devices that have a display very closeto the touch sensors.

FIG. 3 is a portion of a touch sensor array 200 that is configured to beincluded in a thin electronic device, such that the touch sensor array200 is very close to a display panel. The touch sensor includes aplurality of electrodes 202 arranged in rows 204 and columns 206. Thissimplified view of the touch sensor includes three rows 208, 210, 212and three columns 214, 216, 218. The rows may be referred to astransmitter channels TX while the columns may be referred to as receiverchannels RX.

In an actual device, there are significantly more rows and columns. Therows and columns each include linearly aligned, diamond shapedelectrodes. Alternative shapes for the electrodes are envisioned. Therows and column overlap each other so that an area near an intersectionof the rows and columns is configured to detect capacitance betweenadjacent row and column electrodes. Ultimately, a change in capacitancecan be detected and interpreted to respond to a user's selection ortouch.

Circuitry 220 receives analog current signals 221 from each of theplurality of columns. The signals are affected by which one of the rowsor transmitter channels are activated. The circuitry 220 is directed tocompensating for the noise with a voltage by converting the currentsignals to a voltage and removing the noise. In particular, thecircuitry 220 includes a plurality of current to voltage amplifiers 222.The current to voltage amplifiers 222 include an operational amplifier224 that receives the current signals in its inverting input 226. Anon-inverting input 228 of the operational amplifier 224 is coupled toground. An output 232 of the operational amplifier is coupled to acapacitor 230, which is coupled to the inverting input of theoperational amplifier. There is a current to voltage amplifier 222 foreach column of electrodes 206, i.e. a current to voltage amplifier foreach receiver channel.

The circuitry includes at least one additional, noise-reducing currentto voltage amplifier 236. That is there is at least one more current tovoltage amplifier in the system than there are columns of electrodes. Ifthere are n columns, then there are at least n+1 current to voltageamplifiers.

The additional, noise-reducing current to voltage amplifier 236 iscoupled to one of the rows of electrodes (one of the transmitterchannels). In FIG. 3, the current to voltage amplifier 236 is coupled tothe third row 212. The current to voltage amplifier receives a current238 from this row that is indicative of the overall noise from thedisplay, or parasitic capacitance caused by pixels of the display. Thiscurrent represents noise across the whole system.

This additional current to voltage amplifier 236 includes an operationalamplifier 240 that has an inverting input 242 that receives the signal238 and a non-inverting input 246 that is coupled to ground. A capacitor248 is coupled between an output 250 of the operational amplifier andthe inverting input 242. The output 250 is a voltage representative ofnoise in the touch sensor system. The output 250 is combined with eachof the voltage outputs of the current to voltage amplifiers 222 that arecoupled to the columns of electrodes. The voltage from thenoise-reducing circuitry is subtracted or removed from the voltageoutput from the receiver channel circuitry.

Each receiver channel circuitry also includes a differential amplifier252 that receives the output 232 of the column current to voltageamplifier 222 and receives the output 250 of the noise-reducing currentto voltage amplifier 236. An output 254 from each differential amplifieris a voltage representative of that column minus voltage attributed tonoise, i.e., RX₂−V_(NOISE). Accordingly, the output 254 is the signalattributable to the actual capacitance associated with touch on thetouch sensor without noise (or reduced by a fraction of the noise)caused by the display.

The touch sensor array 200 is configured to rotate which row is coupledto the noise-reducing current to voltage amplifier 236 as the touchsensor array 200 cycles through the rows. For example, in a system thathas m rows, a first row 208, TX₀, is activated. For example, a currentsource 251 may be coupled to the first row. A second row 212, physicallyspaced from the first row 208, is coupled to the noise-reducing currentto voltage amplifier 236. The remaining rows, m−2 are all coupled toground, such as row 210, TX₁. The sensor systematically cycles throughand activates one of the rows at a time to detect capacitive touches bythe user. When one of the rows is activated, a second one, spaced fromthe first one is coupled to the noise-reducing current to voltageamplifier 236 to remove the noise created by the pixels of the display.The touch screen controller of FIG. 2 is configured to control the touchsensor array and activate and deactivate the rows and columns.

In prior systems, there is no differential measurement associated withone of the rows (transmitter channels) to each of the columns (receiverchannels). In particular, touch detection was performed by activating afirst row, TX₀ and coupling all remaining rows (TX₁ and TX₂) to ground.Next all of the columns RX₀-RX₂ are activated at the same time andmeasurements for each intersection of the activated row TX₀ with eachcolumn are output OX₀-OX₂. This is repeated for each row. A single rowis activated at a time and all of the columns are activated inconjunction with the single row. The capacitance is measured between theactivated row and column electrodes.

As noted above, the present disclosure is directed to activating a firstrow and a second row, where the second row is utilized to remove noise.This method and circuit structure is configured to measure the signaland the noise. The noise is then removed or reduced from each of thereceiver/column channels. Accordingly, by activating one of the rows asa noise detection and reduction channel, separate from the driven row,noise can be detected and subtracted or otherwise removed from thereceiver outputs.

FIG. 4 is directed to an alternative circuit structure to compensate thereceiver outputs with a current as opposed to the voltage compensationdescribed above. FIG. 4 includes a touch sensor array 300 that includesa plurality of electrodes 302 arranged in rows 304 and columns 306.Circuitry 308 is configured to remove noise from the touch sensor array300, such as parasitic capacitance from a coupled liquid crystaldisplay.

As described above, a first rows TX₀ is coupled to a current source 310.A second row TX₂ is coupled to the circuitry 308 and all remaining rowsare coupled to ground, such as TX₁. The second row is physically spacedfrom the first row that is currently coupled to the current source.Current I_(NOISE) from the second row is fed into a first currentconveyor circuit 312. A first input of the current conveyor circuit 312receives the signal I_(NOISE) and a second input is coupled to ground. Acurrent output from each row, for example, I_(SIGNAL), is received as afirst input into a second current conveyor circuit 314. The currentI_(NOISE) is inverted by the current conveyor circuit 312 and then addedto the signal current. This removes the noise from the signal current.The remaining current, indicative of the actual current associated witha user's touch is then processed by a current to voltage converter 316.

The current conveyor 314 outputs a current the same as the inputcurrent. The current conveyor 312 outputs a current that is the inputcurrent inverted, i.e. the sign is changed. The differential current isfed into the next stage and noise is cancelled before the current tovoltage conversion. A similar process is performed for each column toremove noise from each column. In addition, as different rows areactivated for detection of capacitance (as the first row) and the secondrow, spaced from the first row is activated as a noise detection row,the sensor array will cycle through all of the rows to detect where auser has touched the sensor.

The present disclosure allows current touch sensor array designs to beadapted to accommodate for thinner end packaging with a display. Thetouch screen controller drives a first transmitter channel for detectionand activates a second transmitter channel for common mode noisedetection. Differential measurement removes the common mode noise fromthe signals received from the receiver channels. The second transmitterchannel changes with each sweep of the sensor array. This secondtransmitter channel is selected to be far from the driven transmitterchannel to avoid or minimize capacitive coupling from the driventransmitter channel.

Deciding which transmitter channel to activate for detection and whichtransmitter channel to activate for noise reduction can be achieved withpredictive touch. In particular, the first transmitter channel fordetection should be close to where the user's last touch was detected.The second transmitter channel for noise should be spaced from the firsttransmitter channel, i.e. spaced from the most recent touch detected. Ananalog multiplexer, controlled by a digital processor, such as an ASIC,may control which row is activated.

The system may do a preliminary detection of where the user's finger islocated and activate based on the preliminary detection. Alternativelyor in addition, the system may use the latest information about theuser's finger. This preliminary self-sensing may be achieved by using aone dimension sweep, drive one row and one column. Alternatively, allcolumns may be driven at one time to create a one dimensional map. Thiscould be a one dimensional vertical map that is compared to a onedimensional horizontal map. The maps being generated from a verticalsweep of the electrodes and a horizontal sweep of the electrodes.

The present disclosure may include a programmable gain setting. Forexample, some amount of noise should be removed before the processingcircuitry, such as an ASIC (application specific integrated circuit)receives the digital signals after the differential measuring. A noisereduction of 50% or 60% may be selected. Each processing system canhandle a different amount of noise. The gain scaling can provideflexibility of this technique is different applications.

The present disclosure simplifies the digital processing side in theASIC, minimizing the filtering needed because the noise is removed onthe analog side. This can save silicon area on the ASIC, simplifying thechip, and reducing the size of the chip.

The various embodiments described above can be combined to providefurther embodiments. Aspects of the embodiments can be modified, ifnecessary to employ concepts of the various patents, applications andpublications to provide yet further embodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

1. A device, comprising: a touch sensor array that includes: a pluralityof rows, each row having a plurality of electrodes; a plurality ofcolumns of electrodes, each column having a plurality of electrodes; aplurality of first current to voltage amplifiers, each one of theplurality of first current to voltage amplifiers coupled to one theplurality of columns; a plurality of differential amplifiers, each oneof the plurality of differential amplifiers coupled to one of theplurality of first current to voltage amplifiers; and a second currentto voltage amplifier configured to be coupled to one of the plurality ofrows, an output of the second current to voltage amplifier being coupledto each one of the plurality of differential amplifiers.
 2. The deviceof claim 1, further comprising a transparent layer on the touch sensorarray.
 3. The device of claim 2, further comprising a display, the touchsensor array being positioned between the transparent layer and thedisplay.
 4. The device of claim 3 wherein the display includes apolarizing layer and a thin film transistor layer.
 5. The device ofclaim 3 wherein the display is flexible.
 6. The device of claim 3wherein a distance between the display and the touch sensor array isless than 10 microns.
 7. A method, comprising: reducing parasiticcapacitance in a touch sensor from a display, the reducing including:activating a plurality of columns of electrodes; activating a first rowof electrodes of a plurality of rows of electrodes; coupling a secondrow of electrodes to a noise-reducing circuit; and coupling remainingones of the plurality of rows of electrodes to ground.
 8. The method ofclaim 7, further comprising detecting a touch by a user with theplurality of column electrodes.
 9. The method of claim 8, furthercomprising determining a third row of electrodes to activate based on alocation of the touch.
 10. A system, comprising: a display; a touchsensor array coupled to the display, the touch sensor array including: aplurality of rows of electrodes; a plurality of columns of electrodes;and a plurality of amplifiers, a first one of the plurality ofamplifiers coupled to one of the plurality of rows of electrodes andeach remaining amplifier coupled to one of the plurality of columns ofelectrodes.
 11. The system of claim 10 wherein the touch sensor arrayincludes a plurality of differential amplifiers, each differentialamplifier coupled to the first one of the plurality of amplifiers. 12.The system of claim 11 wherein each differential amplifier is coupled toone of the remaining amplifiers that are coupled to the ones of theplurality of columns of electrodes.
 13. The system of claim 10, furthercomprising a touch sensor controller configured to activate anddeactivate the plurality of rows and columns of electrodes of the touchsensor array.
 14. The system of claim 10, further comprising anapplication specific integrated circuit coupled to the remainingamplifiers that are coupled to one of the plurality of columns ofelectrodes.
 15. The system of claim 10 wherein the display is a liquidcrystal display.